As the result of continuous advances in technology, particularly in the area of networking, such as the Internet, there is an increasing demand for communications bandwidth. For example, the transmission of images or video over the Internet, the transfer of large amounts of data in transaction processing, or videoconferencing implemented over a public telephone network typically require the high speed transmission of large amounts of data. As applications such as these become more prevalent, the demand for communications bandwidth will only increase.
Optical fiber is a transmission medium that is well suited to meet this increasing demand. Optical fiber has an inherent bandwidth much greater than metal-based conductors, such as twisted-pair or coaxial cable; and protocols such as Synchronous Optical Networking (SONET) have been developed for the transmission of data over optical fibers.
Optical fiber is used to form optical networks that carry data, voice, and video using multiple wavelengths of light in parallel. Light is routed through the network from its originating location to its final destination. Since optical networks do not generally have a single continuous optical fiber path from every source to every destination, the light is switched as it travels through the optical network. Previously, this switching was accomplished using optical-electrical-optical (xe2x80x9cOEOxe2x80x9d) systems, where a light signal was converted to an electrical signal, switched electrically, and then output optically. Because in OEO systems the signal must be converted from optical to electrical, switched, then converted back to optical, OEO systems are relatively large, complex, and expensive. More seriously, the OEO systems are slower than purely optical systems, and consequently introduce undesirable bottlenecks.
Much effort is being expended on the development of all-optical cross-connect switching systems, some of which employ arrays of electrostatically, electromagnetically, piezoelectrically, or thermally actuated mirrors. Digitally controlled mirrors with on and off states can be used to switch between small numbers of ports while analog controlled mirrors can be implemented with a small or a large number of ports. Analog controlled mirrors require bi-axial actuation; unfortunately, most electrostatic actuators used to position these mirrors suffer from relatively low torque, and consequentially require relatively high supply voltages to produce sufficient motion. The lack of torque also renders electrostatic actuators very sensitive to vibrations. There is therefore a need for a bi-axial actuator that operates at lower voltages and is relatively insensitive to vibration.
The invention is directed to Micro-Electro-Mechanical Systems (MEMS) actuators that employ electrostatic comb electrodes to position mirrors along multiple axes. In one embodiment, an actuator assembly includes an actuator support, typically a silicon wafer, supporting one or more fixed comb-shaped electrodes, each with a plurality of teeth. A frame flexibly connected to the actuator support includes complementary sets of movable comb electrodes, the teeth of which are arranged interdigitally with the teeth of the fixed combs. The frame can be tilted with respect to the actuator support along a first fulcrum axis by applying a potential difference between the fixed and movable combs.
Each actuator assembly also includes an actuated member flexibly connected to the frame. In the depicted embodiment, the actuated member is a mirror mount. In other embodiments, the actuated member may support e.g. a filter, a lens, a grating, or a prism.
The actuated member and the frame include electrically isolated, interdigitated, comb electrodes. The actuated member can be moved relative to the frame along a second fulcrum axis by applying a potential between the comb on the frame and the comb on the actuated member. The actuated member can also be moved translationally by applying a potential between interdigitated combs.
In one embodiment, the hinges are made using the same conductive layers as the combs. The process used to form the hinges may differ from the process used to form the combs. Such processes allow the stiffness of the hinges to be adjusted independently. For example, the hinges may be made thinner to reduce the amount of torque required to move the actuated member. In another embodiment, serpentine hinges are employed to provide still greater flexibility.
A number of novel process sequences can be employed to manufacture MEMS actuators in accordance with the invention. In one such process, referred to herein as a xe2x80x9cwafer bondingxe2x80x9d process, one device layer on a Silicon-On-Insulator (SOI) or Spin-On-Glass (SOG) wafer is patterned to include the combs, hinges, etc., of the MEMS actuator(s) being formed. This patterned layer is then oxide- or glass-bonded to an intrinsic anchor wafer. A via etching is then performed on the other side of the intrinsic anchor wafer to electrically connect the devices to the driving circuitry. The other side of the original SOI or SOG wafer is then ground, polished, patterned, and etched as another device layer. Up to four different thicknesses are defined in these lithographic processes.
In another process, referred to herein as a xe2x80x9cpattern transferxe2x80x9d process, one device layer is patterned to include features similar to the combs, hinges, etc., of the MEMS actuators being formed. The resulting pattern is then xe2x80x9ctransferredxe2x80x9d to the surface of a second material layer by etching the top surface of the first material layerxe2x80x94including the raised portions and the valleys defined between the raised portions xe2x80x94until the second layer is exposed between the raised portions.
A third process that can be used to form MEMS actuators in accordance with the invention, referred to herein as xe2x80x9cdeep-well lithography,xe2x80x9d differs from conventional lithography in that the surface being patterned is not the uppermost surface. The focal plane of the photolithography equipment is offset from the uppermost surface as appropriate to account for the depth of the well in which the pattern is to be formed.
Both the pattern-transfer process and deep-well lithography advantageously reduce the number of process steps required to produce MEMS actuators in accordance with the invention, and can additionally be used to form structures other than MEMS actuators.
This summary does not limit the invention, which is instead defined by the appended claims.